In harvesting solar energy, it may be desirable to orient equipment so as to better receive the sun's rays. For example, the energy output of photovoltaic panels may be increased by orienting them to face the sun more directly as it moves across the sky. Such orientation may be desirable for the operation of solar equipment including optics to concentrate sunlight, for example, for the purpose of generating heat, or for the purpose of increasing photovoltaic cell output. Such orientation may also be desirable for the operation of heliostats, in which mirrors are oriented to direct sunlight to a tower. Alternatively, in the case of a radio antenna, orientation of a directional radio antenna may be required in order to maximize a desired signal from a radio source in the sky or to effectively communicate with a satellite or a space probe.
In a solar energy application, the orientation relative to the sun must be to an accuracy that depends on the type of supported equipment. An error of ten or more degrees may be acceptable for flat solar panels, while a few tenths of a degree of accuracy may be needed for optical concentrating equipment. Similarly, a few tenths of a degree of accuracy may be required for a high frequency radio antennae. The desired accuracy of orientation must be maintained during operation despite changes in gravitational forces resulting from varying orientations of the attached solar apparatus or antenna, or in the face of wind forces which may become larger than the force of gravity. In extreme weather the tracker may be subject to downbursts and gale force winds with a vortex or whirling component or a lift component. The rotating support must thus be able to resist strong lateral as well as vertical forces acting on the supported equipment, and large torques or bending moments about any axis.
A further constraint on rotating apparatus in some cases will be clearance to allow downward tilt of supported solar equipment, for example, a range of motion needed to follow the sun toward the horizon late in the day.
The designer of a rotating apparatus and support for a solar energy application is presented with the challenge of meeting the above technical requirements economically. For example, the marketable energy output of photovoltaic panels may be increased by around 35% if they are oriented to face the sun directly all day (as compared to static mounting), but this will be worthwhile only if the additional initial expense of manufacture and installation of a sun-tracker can be recovered through return on the additional energy generated. Thus low cost of manufacture, installation and maintenance of solar tracking equipment is an essential requirement for an apparatus to support and rotate solar equipment.
Sun-trackers can incorporate a mechanism to provide rotation about a vertical axis fixed with respect to the ground. In such an example, the equipment rotated may include solar photovoltaic panels or solar reflectors. Additional mechanisms can be included to provide rotation about a second axis. In the past, a post anchored in a ground foundation has been employed surmounted by a rotatable section to provide motion about a vertical axis (azimuth). The mechanical load acting on the supported equipment would be transmitted through the rotatable section and down the fixed post to the ground foundation. The fixed post and the rotatable section would be typically linked by one or more bearings which constrain the motion to be about the vertical axis. Rotation would be accomplished by means of a driven mechanical coupling reacting against the top of the fixed post.
In other systems, motion about a fixed azimuth axis would be provided by a bearing and platform close to the ground, or atop a tripod. Yet other forms of a sun-tracker have provided for motion in two dimensions about a universal joint rather than about specific axes.
In the past, sun-tracking systems that provide for motion about more than one axis have not been economical enough for widespread adoption. Sun-tracking systems that provide for motion about a single horizontal axis have been more economical, but nevertheless still leave room for improvement. In such systems, the horizontal axis may be defined by bearings atop a line of posts. In the past, sun-tracking systems that provide for rotation about a vertical axis have commonly been heavy and expensive. Forces and torques acting on the equipment being turned are typically reacted by large forces acting across short distances, driving up weight and cost. For example, commonly the action of lateral wind force on equipment is taken at the top of a cantilevered fixed post, resulting in amplified forces of compression and tension at the root of the fixed post. The walls of such a fixed post at its base must thus be thick and heavy to avoid failure in high wind. Similarly, the foundations supporting such a fixed post are typically massive to prevent mechanical failure at the ground attachment.
Mechanisms to drive azimuth motion also contribute to high weight and cost. Those used on top of fixed posts or other pedestals are typically constrained to be much smaller in size than the equipment they support, so as not to interfere with the full range of elevation motion of the solar apparatus. As a consequence of small size, torques on the solar apparatus from gusting wind translate into high forces at the drive mechanism, requiring use of heavy steel drive parts to avoid damage, which are relatively expensive and drive up cost.